The Scottish Society for the Prevention of Cruelty to Animals (SSPCA) said it received 22 reports yesterday of dogs trapped in vehicles in the sweltering heat compared to last year's average of nine a day.
Temperatures reached 31.9C in Glasgow today while it was 33C at Porthmadog in North Wales, making it the hottest day of the year so far.
It has had a total of 521 calls so far this year - 41 per cent up on the same period in 2017.
Police yesterday freed a dog overheating in a car at Leuchars in Fife.
They tweeted: "We’ve just been forced to smash a car window to free a dog from a hot car.
"Officers in the car state it was 'boiling'.
"The dog has been given water & has been taken to the vets. We will be taking further action."
Officers added later: "He’s been checked over by a vet who has no further concerns about his welfare.
"The owner has been traced and charged with causing an animal unnecessary suffering. A report will be submitted to the procurator fiscal."
They said pug was the breed of dog most at risk of overheating, followed by pekinese, bulldog and shihtzu.
SSPCA chief superintendent Mike Flynn said: “Many people might not realise that even on warm, cloudy days, cars can effectively turn into ovens and dogs can overheat in minutes.
“Leaving a window open and providing water is not enough.
"Being left inside a hot vehicle causes dogs to pant to try to cool down and this can lead to severe dehydration and hyperventilation, which is extremely dangerous.
“Our advice is that owners should leave their dogs at home if they are going to be left in the car for any length of time at all.
“Anyone who comes across a distressed dog left in a vehicle should contact our animal helpline on 03000 999 999 or police.”